Control valves, as the type of final control elements in question, are generally known in automation and process engineering as extremely important elements in the control and regulation of processes. Their reliability is a crucial factor in the quality of the overall control process. Faults occurring during operation can result in failure of the entire system, with high maintenance costs as a consequence. Hence early diagnosis and thereby detection of faults in the valve can prevent such failures, and consequently also reduce the costs that arise from replacing, as a precaution, valves that are still working perfectly.
In particular, leaks from valves in the closed state are of significant diagnostic interest. The sealing action of the valve seating is reduced by ageing processes or dirt accumulation, and the process medium continues to flow through the valve despite a closed valve being signaled to the outer world.
Such leaks can be detected, for example, by a flow-rate sensor connected downstream that is installed additionally in the process line. Such a sensor is very expensive, however, and there is a high cost involved in fitting the sensor. In addition, the power consumption of the flow-rate sensor is normally so high that it cannot share the supply for the valve controller, but requires an additional supply line. Thus such a sensor is also normally only installed when it is already required for the process control system. In addition, it is known from the dissertation of Sebastian Maria Mundry “Zustandsüberwachung an Prozessventilen mit intelligenten Stellungsreglern” [“Monitoring the status of process valves using intelligent positioners”], Shaker Verlag, Aachen, 2002, that flow-rate sensors for measuring the maximum flow rate are not suitable for reliable detection of the low flow rates from leaks.
It is also known from the same publication that the flow of a fluid under pressure through a narrow aperture produces an acoustic signal as a result of various physical effects. For instance, the high flow rates that arise cause severe turbulence after the aperture, and the pressure drop in the flow results in cavitation. The turbulence and the collapse of the cavitation bubbles produce an acoustic signal that is directly dependent on the flow rate and the fluid properties. At low rates, the signal is composed of individual acoustic pulses generated by the collapse of the individual cavitation bubbles, and develops into white noise at high rates. This acoustic signal is overlaid by the general process sounds in the plant, which are produced by pumps, general flow noises, chemical processes etc.
As these process sounds propagate in the pipeline system of the plant, the sounds are attenuated by different amounts depending on their frequency. The high frequencies, in particular, are strongly attenuated, so that generally process sounds are only detectable at the valve as low-frequency acoustic signals (in the kHz range). Thus the sounds produced by the leak can be discriminated from the general process sounds by measuring in higher frequency ranges. It is known from Leak Detection Service, Maintaining a Successful Valve and Trap Leak Detection Program using the Valve-Analyser System, The 10th Annual Predictive Maintenance Technology National Conference, Nov. 9-12, 1998, that valve service companies currently use ultrasound sensors in order to measure the acoustic signals directly at the valve, i.e. close to the noise source. In addition, these signals are also compared with the signals from ultrasound sensors installed further upstream and downstream in the pipeline system. A leak can then be detected from these signals, and, with suitable calibration, it is even possible to determine the size of the leak for the valve from the signal level.
Furthermore, EP 1216375 B1 and WO 00/73688 A1 disclose detecting the structure-borne noise on the valve casing or on parts directly connected to this, and supplying this information to the positioner, in which it is evaluated and processed. The valve is continuously monitored, with the electronics and position signal already available in the positioner being shared for the diagnosis. The publications also disclose that high-frequency signals (>50 kHz) are analyzed, and that the ultrasound spectrum in the closed state is compared with a signal in the slightly open state. The latter method can be applied equally well to reducing the ambient noise without comparative measurements needing to be made at different points in the direction of flow and against the direction of flow. Although sharing the use of the position-sensor electronics and the position signal does reduce the installation costs for this diagnostic system, the ultrasound sensor head itself must still be fitted on the valve as an additional external unit.